The present invention pertains to a lighting system and, more particularly, to a system which is readily adapted to provide emergency lighting in a new installation, or to be incorporated in an already existing lighting installation by retrofitting. In the latter case, substantially no extra wiring is required.
The lighting system of the present invention is concerned with supplying power on an emergency basis due to power outages, for example, to electron discharge devices, such as fluorescent lamps or the like, whereby the same fluorescent lamps that function in an emergency also provide illumination in normal circumstances; that is to say, when no emergency exists.
Prior art systems, and specifically those that contemplate the supply of power to fluorescent lamps or fluorescent fixtures, have taken a variety of approaches in the attempt to achieve efficient operation. One such system, which has the objective of providing emergency power by means of the same wiring that is used to provide the utility power to fluorescent lights, is that disclosed in U.S. Pat. No. 4,056,757. That system eschews the kinds of schemes which involve locating batteries and associated components in or near the light fixture, since it concludes that the components would thereby be operating in a hostile environment, with the very significant degrading factor being the well-known sensitivity of batteries to heat. That same patent also notes that with such schemes some additional wiring, other than that needed to operate from the utility power, is required, and such additional wiring is expensive and may be inconvenient to provide.
Another approach that is followed in certain prior art emergency lighting systems involves locating components needed for the emergency lighting remote from the light fixtures, in which case separate wiring is required to transmit emergency power to the light fixtures. Such an approach results in inefficiencies because the additional extended wiring adds considerably to the expense of the system, and is inconvenient to install, particularly in existing buildings.
Whatever the merits of the system disclosed in U.S. Pat. No. 4,056,757, and these involve the aforenoted advantage of providing the emergency power by means of the same wiring used to provide utility power, there remains the fact that such system relies on the operation of a central inverter. However, such inverter units are very expensive and are trouble-prone. Moreover, special ballasts are required to discriminate between the normal supply condition, that is, when utility power is available, and the emergency power condition such that only selected fixtures will be illuminated when the latter condition prevails.
Accordingly, it is a primary object of the present invention to overcome the noted drawbacks and difficulties normally found in prior art systems.
To provide further background for consideration of the present invention, reference may also be made to the following: U.S. Pat. No. 3,448,335 in which a high frequency AC-DC fluorescent lamp driver circuit is described; U.S. Pat. No. 3,356,891 in which an automatic substitution of a standby power source is made operative only when particular fluorescent lamps are connected in circuit; U.S. Pat. No. 3,684,891 in which a localized battery emergency power supply system is described.
Broadly stated, the present invention provides a lighting system in which selected ones of the lighting fixtures, used for providing normal illumination from an AC utility power supply, can likewise be used to furnish illumination under emergency conditions from a DC power supply. The system includes an individual inverter power supply for substantially continuously supplying high frequency power to each of the fluorescent light fixtures involved. Moreover, each of the inverters supplies high frequency power, i.e., of the order of 20 KHz, whether it be the AC supply or the emergency DC supply that is connected to the input of the inverters. A transfer means effects the changeover whereby only selected lighting fixtures function for emergency lighting purposes. In a preferred embodiment, a switching means is connected to the branch circuit or circuits having the particular ones of the lighting fixtures that will provide the emergency lighting, and the DC power supply is selectively connected to only these branch circuits, transmission of DC being accomplished by the same conductors that ordinarily furnish AC. A rectifier is included at the input to each inverter to convert the incoming AC, when present, to DC, which is the appropriate input supply for the inverter.
In accordance with a specific embodiment of the present invention, a typical system would involve the lighting of a large-scale area, for example, requiring on the order of 100 fixtures and each typically having two fluorescent lamps or lights. Such large-scale installation would include a number of branch circuits, for example, ten branch circuits, each supplying ten fixtures. One of these branch circuits would be an emergency lighting branch circuit and could also be used alternatively as a "night light" circuit which typically functions twenty-four hours a day under utility power. This emergency branch circuit would supply suitably spaced emergency fixtures so that the entire room would receive complete illumination, but, of course, at a much reduced wattage level.
It will therefore be appreciated that the present invention enables the functioning of all fluorescent fixtures under normal circumstances and permits a diffused low level emergency illumination from selected ones of the very same fixtures which provide the normal illumination.
Each of the individual inverters associated with each of the fluorescent fixtures may be of the type disclosed in U.S. Pat. No. 4,017,785, the disclosure of which is incorporated herein by reference. Such an inverter operates to supply high frequency power to a pair of fluorescent lamps at the output of the inverter. This frequency is usually selected to be above 20,000 Hz. Such an inverter or "electronic ballast" enables an increase in efficiency of a given fixture of approximately 25% and is therefore ideal for the conservation of energy in present day circumstances.
Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawing, wherein like parts have been given like numbers.